The plastic sheet can be difficult to find, but is available at home depot.  It is in the wall paneling section as it is intended to be glued to commercial bathroom walls.

It comes in 4'x8' sheets for about $20 USD.  This is the 0.060" thickness, and it does not have the fiber reinforcing, making it much more flexible.  That is, you do not want the FRP version.  Home Depot does carry this, but it does not appear to be on their website.  Lowes carries it as well, and lists it on their website, link in the components list.

I made a copy of my tail pattern, with just the fluke part, and you can see how it will fit on the monofin:

I used pattern weights to trace it onto the plastic.  I use ceramic toroids from large inductors for my pattern weights as they have a nice coating which prevents marking, good weight, and easy to pick up.

To cut the plastic, I started by using metal shears as I had both left and right curved ones.  They did work, but were more awkward than I needed, and I later switched to just using utility scissors, which worked fine.

That plastic doesn't lay flat because of the re-enforcing bars on the monofin.  I will adjust this by cutting slots out of the plastic to match.  

Because 4 of the re-enforcing bars go pretty close to the trailing edge, I decided to trim some of the bars flush, simply using a strong wire cutters.

This results in a flush fit

As can be seen above, a small portion of the monofin protrudes out into the split in the fluke.  I simply used a utility knife to cut it back a bit.  The monofin is 1/8" thick, and probably HDPE, so wasn't too difficult to cut.

I repeated this process for the backside as well.  Next up is to attach the plastic.